(function() {
    'use strict';

    var p = Element.prototype;
    var matches = p.matches || p.matchesSelector || p.mozMatchesSelector ||
      p.msMatchesSelector || p.oMatchesSelector || p.webkitMatchesSelector;

    Polymer.IronFocusablesHelper = {

      /**
       * Returns a sorted array of tabbable nodes, including the root node.
       * It searches the tabbable nodes in the light and shadow dom of the chidren,
       * sorting the result by tabindex.
       * @param {!Node} node
       * @return {!Array<!HTMLElement>}
       */
      getTabbableNodes: function(node) {
        var result = [];
        // If there is at least one element with tabindex > 0, we need to sort
        // the final array by tabindex.
        var needsSortByTabIndex = this._collectTabbableNodes(node, result);
        if (needsSortByTabIndex) {
          return this._sortByTabIndex(result);
        }
        return result;
      },

      /**
       * Returns if a element is focusable.
       * @param {!HTMLElement} element
       * @return {boolean}
       */
      isFocusable: function(element) {
        // From http://stackoverflow.com/a/1600194/4228703:
        // There isn't a definite list, it's up to the browser. The only
        // standard we have is DOM Level 2 HTML https://www.w3.org/TR/DOM-Level-2-HTML/html.html,
        // according to which the only elements that have a focus() method are
        // HTMLInputElement,  HTMLSelectElement, HTMLTextAreaElement and
        // HTMLAnchorElement. This notably omits HTMLButtonElement and
        // HTMLAreaElement.
        // Referring to these tests with tabbables in different browsers
        // http://allyjs.io/data-tables/focusable.html

        // Elements that cannot be focused if they have [disabled] attribute.
        if (matches.call(element, 'input, select, textarea, button, object')) {
          return matches.call(element, ':not([disabled])');
        }
        // Elements that can be focused even if they have [disabled] attribute.
        return matches.call(element,
          'a[href], area[href], iframe, [tabindex], [contentEditable]');
      },

      /**
       * Returns if a element is tabbable. To be tabbable, a element must be
       * focusable, visible, and with a tabindex !== -1.
       * @param {!HTMLElement} element
       * @return {boolean}
       */
      isTabbable: function(element) {
        return this.isFocusable(element) &&
          matches.call(element, ':not([tabindex="-1"])') &&
          this._isVisible(element);
      },

      /**
       * Returns the normalized element tabindex. If not focusable, returns -1.
       * It checks for the attribute "tabindex" instead of the element property
       * `tabIndex` since browsers assign different values to it.
       * e.g. in Firefox `<div contenteditable>` has `tabIndex = -1`
       * @param {!HTMLElement} element
       * @return {!number}
       * @private
       */
      _normalizedTabIndex: function(element) {
        if (this.isFocusable(element)) {
          var tabIndex = element.getAttribute('tabindex') || 0;
          return Number(tabIndex);
        }
        return -1;
      },

      /**
       * Searches for nodes that are tabbable and adds them to the `result` array.
       * Returns if the `result` array needs to be sorted by tabindex.
       * @param {!Node} node The starting point for the search; added to `result`
       * if tabbable.
       * @param {!Array<!HTMLElement>} result
       * @return {boolean}
       * @private
       */
      _collectTabbableNodes: function(node, result) {
        // If not an element or not visible, no need to explore children.
        if (node.nodeType !== Node.ELEMENT_NODE || !this._isVisible(node)) {
          return false;
        }
        var element = /** @type {!HTMLElement} */ (node);
        var tabIndex = this._normalizedTabIndex(element);
        var needsSort = tabIndex > 0;
        if (tabIndex >= 0) {
          result.push(element);
        }

        // In ShadowDOM v1, tab order is affected by the order of distrubution.
        // E.g. getTabbableNodes(#root) in ShadowDOM v1 should return [#A, #B];
        // in ShadowDOM v0 tab order is not affected by the distrubution order,
        // in fact getTabbableNodes(#root) returns [#B, #A].
        //  <div id="root">
        //   <!-- shadow -->
        //     <slot name="a">
        //     <slot name="b">
        //   <!-- /shadow -->
        //   <input id="A" slot="a">
        //   <input id="B" slot="b" tabindex="1">
        //  </div>
        // TODO(valdrin) support ShadowDOM v1 when upgrading to Polymer v2.0.
        var children;
        if (element.localName === 'content' || element.localName === 'slot') {
          children = Polymer.dom(element).getDistributedNodes();
        } else {
          // Use shadow root if possible, will check for distributed nodes.
          children = Polymer.dom(element.root || element).children;
        }
        for (var i = 0; i < children.length; i++) {
          // Ensure method is always invoked to collect tabbable children.
          needsSort = this._collectTabbableNodes(children[i], result) || needsSort;
        }
        return needsSort;
      },

      /**
       * Returns false if the element has `visibility: hidden` or `display: none`
       * @param {!HTMLElement} element
       * @return {boolean}
       * @private
       */
      _isVisible: function(element) {
        // Check inline style first to save a re-flow. If looks good, check also
        // computed style.
        var style = element.style;
        if (style.visibility !== 'hidden' && style.display !== 'none') {
          style = window.getComputedStyle(element);
          return (style.visibility !== 'hidden' && style.display !== 'none');
        }
        return false;
      },

      /**
       * Sorts an array of tabbable elements by tabindex. Returns a new array.
       * @param {!Array<!HTMLElement>} tabbables
       * @return {!Array<!HTMLElement>}
       * @private
       */
      _sortByTabIndex: function(tabbables) {
        // Implement a merge sort as Array.prototype.sort does a non-stable sort
        // https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/sort
        var len = tabbables.length;
        if (len < 2) {
          return tabbables;
        }
        var pivot = Math.ceil(len / 2);
        var left = this._sortByTabIndex(tabbables.slice(0, pivot));
        var right = this._sortByTabIndex(tabbables.slice(pivot));
        return this._mergeSortByTabIndex(left, right);
      },

      /**
       * Merge sort iterator, merges the two arrays into one, sorted by tab index.
       * @param {!Array<!HTMLElement>} left
       * @param {!Array<!HTMLElement>} right
       * @return {!Array<!HTMLElement>}
       * @private
       */
      _mergeSortByTabIndex: function(left, right) {
        var result = [];
        while ((left.length > 0) && (right.length > 0)) {
          if (this._hasLowerTabOrder(left[0], right[0])) {
            result.push(right.shift());
          } else {
            result.push(left.shift());
          }
        }

        return result.concat(left, right);
      },

      /**
       * Returns if element `a` has lower tab order compared to element `b`
       * (both elements are assumed to be focusable and tabbable).
       * Elements with tabindex = 0 have lower tab order compared to elements
       * with tabindex > 0.
       * If both have same tabindex, it returns false.
       * @param {!HTMLElement} a
       * @param {!HTMLElement} b
       * @return {boolean}
       * @private
       */
      _hasLowerTabOrder: function(a, b) {
        // Normalize tabIndexes
        // e.g. in Firefox `<div contenteditable>` has `tabIndex = -1`
        var ati = Math.max(a.tabIndex, 0);
        var bti = Math.max(b.tabIndex, 0);
        return (ati === 0 || bti === 0) ? bti > ati : ati > bti;
      }
    };
  })();